1. Field of the Invention
The present invention relates to a soft magnetic steel part useful for forming iron cores for solenoids, relays and solenoid valves to be applied to various electric devices typically for automobiles, electric trains and ships, and a soft magnetic steel as a material for the soft magnetic steel part. More particularly, it relates to a soft magnetic steel that can yield a steel part with excellent dimensional accuracy in a high yield by shape forming (hereinafter this property is also simply referred to as “cold forgeability”), exhibits satisfactory machinability in production of the part by machining, and ensures excellent magnetic properties meeting requirements specified in Japanese Industrial Standards (JIS) SUYB Class 1 or higher as a result of magnetic annealing. It also relates to a soft magnetic steel part that is made from the steel and has excellent magnetic properties meeting requirements specified in JIS SUYB Class 1 or higher.
“SUYB” herein represents a standard of magnetic properties specified in JIS C 2503. The electric devices require magnetic properties approximately at such a level meeting requirements specified in JIS SUYB Class 1.
2. Description of the Related Art
Magnetic circuits included in electric devices typically for automobiles are required to be more precisely controlled for the improvement of power consumption and magnetic responsibility of the electric circuits, responding to the purpose of energy savings, typically in automobiles. The material steels for the electric devices are required to have a low coercive force, in addition to a capability of being easily magnetized by a low-intensity external magnetic field.
Thus, those electric devices are generally formed of soft magnetic steels so that the magnetic flux density in the electric devices changes in quick response to the change of an external magnetic field. Representative soft magnetic steels are very-low-carbon steels having a carbon (C) content on the order of 0.01 percent by mass or less (a soft magnetic pure-iron-based material). An electric device (a soft magnetic steel part) is generally manufactured by subjecting a steel billet of a very-low-carbon steel to hot rolling, subjecting the resulting steel sheet typically to lubrication and drawing to yield a steel wire, and sequentially subjecting the steel wire to forming work (cold forging) and magnetic annealing.
Electric devices using electromagnetic force have conventionally chiefly used as switches typically for hydraulic control in various fields such as the auto industry field. However, a control system of directly driving mechanisms by the action of electromagnetic force is now increasingly employed for power saving and higher performance. Electric devices having this control system require higher electromagnetic driving force than conventional electric devices, to which a magnetic field at a high intensity of 5000 A/m or more is applied. Accordingly, a soft magnetic steel part that stably exhibits excellent magnetic properties even in magnetic fields at such high intensities, and a soft magnetic steel as a material for the soft magnetic steel part have been demanded.
The dimensions of, for example, iron cores of such electromagnetic parts have become increased in size and complicated more and more. In this connection, iron cores requiring excellent magnetic properties must not only have excellent magnetic properties of their materials but also undergo finish machining with high precision, because trace variations in dimensions of produced parts significantly adversely affect the magnetic properties of the final products. Avery-low-carbon steel (soft magnetic pure-iron-based material) is flexible and resistant to cutting. Consequently, the productivity markedly decreases when a very-low-carbon steel part with high dimensional accuracy is to be manufactured by machining.
A possible solution to improve the machinability of soft magnetic pure-iron-based materials can be found in, for example, Japanese Unexamined Patent Application Publication (JP-A) No. 2003-055745. This technique is intended to minimize the reduction in magnetic properties due to elements that impart free-machinability and to inhibit burrs during machining to thereby improve the productivity, by controlling the distribution and dimensions of MnS grains in the steel within proper ranges. However, the technique is still susceptible to improvement in variation of properties when the steel is manufactured in a continuous annealing system.
Techniques for reducing the influence of eddy current in very-low-carbon steels can be found in JP-A No. 2000-8146 and JP-A No. 2000-30922. These techniques are mainly intended to reduce eddy-current loss in an alternating magnetic field by controlling the dispersion of sulfides in steels, but they fail to consider steels for use in applications requiring excellent magnetic properties in a high-intensity magnetic field, as in electromagnetic solenoids.